Doppel(Dpl)is a prion(PrP)-like protein due to the structural and biochemical similarities;however,the natural functions of Dpl and PrP remain unclear.In this study,a 531-bp human PRND gene sequence encoding Dpl protein was amplified from human peripheral blood leucocytes.Full-length and various truncated human Dpi and PrP proteins were expressed and purified from Escherichia coll.Supplement of the full-length Dpl onto human neuroblastoma cell SH-SY5Y induced remarkable cytotoxicity,and the region responsible for its cytotoxicity was mapped at the middle segment of Dpl [amino acids(aa)81-122].Interestingly,Dpl-induced cytotoxicity was antagonized by the presence of fulllength wild-type PrP.Analysis on fragments of PrP mutants showed that the N-terminal fragment(aa 23-90)of PrP was responsible for the protective activity.A truncated PrP(PrPA32-121)with similar secondary structure as Dpl induced Dpl-like cytotoxicity on SHSY5Y cells.Furthermore,binding of copper ion could enhance the antagonizing effect of PrP on Dpl-induced cytotoxicity.Apoptosis assays revealed that cytotoxicity induced by Dpl occurred through an apoptotic mechanism.These results suggested that the function of Dpi is antagonistic to PrP rather than synergistic.